Offshore structures, such as floating production, drilling or construction platforms or other vessels, are moored in the desired location through the use of chains and/or cables extending between the platform and anchors on the ocean floor. Typically, the method for mooring floating platforms includes extending a chain in a catenary from the ocean anchor to a platform, through a fairlead device secured near the bottom of a platform column, to chain hauling equipment and a chain stopper on the deck of the platform. These elements are used to apply the desired mooring tension and to withstand the higher tensions that may be encountered in weather situations.
Mooring platforms in place at a drilling or production location usually require the presence of multiple chains, fairlead devices, anchors and chain equipment because of the massive size of the platforms. These all compete for space on the limited deck area of a platform, which also usually must be large enough for one or more buildings for housing workers and machinery, one or more cranes, and a drilling tower or production facilities.
Floatation of offshore platforms is often provided by large submerged pontoons. Large diameter columns extend upward from the pontoons to support the deck, and the mooring lines are led out from multiple columns. Thus, fairlead devices are usually secured to the columns of the platform below the waterline. For other vessels that are moored in place, the fairlead may be secured to a hull surface or structure extending from the main surface of the hull, also usually, but not exclusively, below the waterline. The mooring lines, often chains or combinations of wire rope and chain, pass from the anchors, through each of the fairlead devices, to line hauling equipment situated on the deck above.
In a typical installation, the anchor lines are installed by passing a messenger line (i.e., installation wire rope) from the deck, down through the submerged fairlead, mounted on a support column, and out to a pre-installed anchor line secured to the ocean floor. An end connector secures the messenger line to the anchor chain and the anchor chain is hauled back to the platform. The anchor chain passes through the fairlead and continues up to the deck as the chain is hauled in to achieve the desired mooring tension. Thus, one of the requirements of an underwater fairlead is that it be able to pass the chain itself, special connecting links and the messenger line.
Because the chain comes into the fairlead at an angle before ascending essentially vertically to the deck, a sheave is used to change direction. The sheaves used in these chain-mooring applications are usually pocketed wheels, known as wildcats, which receive links of the chain in pockets. This helps reduce the chain stresses in the links resting on the wildcat.
On the deck, the chain hauling equipment pre-tensions the chain up to a predetermined percentage of the chain-breaking load. To relieve the chain hauling equipment of the tension load, a chain stopper or chain latch locks the chain in place at the pre-tension load. In some prior art fairleads, the chain stopper or chain latch is made a part of or connected to the fairlead. In that case, the chain stopper or latch will remain submerged in normal use and during servicing. Thus, it is desirable to have a mechanism that needs little service and is easy to service when required.
There is a need in the art for a fairlead design that is simpler and more reliable than existing designs.